MrDontPlay, the whole point is he wants to mix them with his live ammo, for malfunction training.

As long as he only ever uses it at the range, no problem.

However, if he ever uses them for dry-fire away from the range, there could be a huge problem. Similarly, should he use the gun for carry, and accidentally mix in one of his (purportedly) identical dummy rounds, he could have a self-induced malfunction at a very bad time.

I don't see the advantage to these. Let somebody else load the magazines, and use snap caps, when he wants to do malfunction drills.